1. Field of the Invention
This invention relates to an electrical means to measure the stoichiometric ratio of the concentrations of oxygen and other oxidizing gaseous species to the concentrations of various reducing gaseous species such as hydrocarbons, hydrogen and carbon monoxide as might be found in the automotive exhaust.
2. Prior Art
High temperature, solid-state, air-to-fuel ratio (A/F) sensors have seen widespread use in the automotive exhaust as the feedback control element used to maintain the A/F at the input to the cylinders of an internal combustion engine near the stoichiometric value so that any oxidizing and reducing species in the exhaust gas can most efficiently be reduced to low concentrations by an exhaust gas catalyst. The sensors actually determine the ratio of the concentrations of oxidizing and reducing species in the exhaust gas. This in turn is proportional, with a different proportionality constant for each type of fuel, to the A/F which is defined as the ratio of the mass of air to the mass of fuel that is introduced to the cylinders.
Current sensors can be conveniently divided into those which have a step-like transfer function at the stoichiometric A/F ratio (such "stoichiometry sensors" have an output which switches from low to high values as the A/F passes from oxidizing (lean) to reducing (rich) conditions at stoichiometry) and those which have a more nearly linear response over a wide range of A/F on both the rich and lean side of stoichiometry. Because the first type of device has a nonlinear transfer characteristic, it is commonly used in an oscillatory or limit-cycle feedback control strategy. The linear transfer characteristic of the latter type of device makes it advantageous for use in the proportional feedback control of A/F.
Most existing A/F sensors use either an electrochemical or a resistive principle. Thus a publication by H. Dietz, W. Haecher and H. Jahnke, in Advances in Electrochemistry and Electrochemical Engineering, Vol. 10, Wiley, N.Y., pg. 1 (1977), describes a solid state electrochemical cell composed, for example, of zirconium dioxide doped with yttrium dioxide, using platinum electrodes, shaped as a cylinder closed at one end with the exterior electrode exposed to the gas of interest while the interior electrode is exposed to a reference atmosphere of fixed oxygen concentration (typically air). In an automotive application this type of cell typically produces an emf between its electrodes of 20 to 30 mV under lean exhaust conditions and 800 to 900 mV under rich conditions with a step like transition occurring near stoichiometry.
Similarly, a publication by E. M. Logothetis, Ceramic Engineering Science Proceedings, 8th Automotive Materials Conference, 1, 281 (1980) describes a solid oxide (e.g. titanium dioxide) device whose resistance changes by several orders of magnitude at the stoichiometric A/F when it is alternately exposed to rich and lean exhaust gas conditions. This change in resistance is often determined using a bridge circuit in which the gas sensitive resistor in one arm of the bridge is used with a thermistor (whose temperature coefficient of resistance matches that of the A/F device but which is insensitive to the gas phase) in another appropriate arm of the bridge to compensate for any changes in resistance which occur due to temperature variations. Both of these stoichiometric A/F sensing principles can be embodied in a number of different materials. Because the electrochemical devices allow the possibility of oxygen pumping, a number of structures have used this process in combination with the measurement of the emf of the same or other cell (e.g. see U.S. Pat. No. 4,272,329 to Hetrick) in a method which enables the A/F measurement over a wide range of values.